Manufacturing of multi-layer preforms with scrap PET

Abstract: The technical field of the invention are the moulding machines, particularly the inner part (core pin) of such moulding machines for manufacturing of thermoplastic preforms, which are later on stretch-blown to containers such as PET-bottles. The invention relates to preforms having a multi-layer structure and incorporating a recycled inner layer, embedded into two layers of virgin material being on the outside and the inside of said preform. The core pin is designed by the invention such that the inner layer of the preform has a minimum thickness at their bottom area and that the minimum thickness is considerably larger than that in a layer known from prior art. The increase of layer-thickness is achieved by designing the core (2) in its front end portion (3a,3b) in an unround fashion. Possible designs of unround front ends are a flat (3b) or a shallow dimple (3a). With such design of the core pin the invention assures that the middle recycled layer does not reach the inner surface of the preform to give the preform - although recycled thermoplasts are used - food-compatibility.

Description: The technical field of the invention are the injection moulding machines, which may produce one or a plurality of multi-layer preforms of thermoplastic material to be further manufactured into one-way or multiway-containers, especially PET-bottles; particularly a certain shape of the inner core (pin) of such machinery and a method to improve the thickness distribution of the inner layer of virgin thermoplastic material is addressed by the invention. A number at layers ("multi-layer") are used for the preform and different thermoplastic melts are successively used and pressed through an injection orifice into a cylindrical cavity. At both surfaces, the outer surface of the inner body and the inner surface of the outer body consolidated layers are established, leaving therebetween enough molten material, through which new molten material of different thermoplastic material may be introduced further up the preform. By such method a multi-layer design is established for the preform, mostly having five layers. Five layers are provided by injecting two different types A,B of the molten thermoplastic material, having the succession A,B,A. An arrangement with an unround front end of the core pin according to the preamble part of claim 1 is shown in WO-A 91/16188 (Beck), specifically being a concavity or a (full) flattening, as e.g. also shown in GB-A 2,010,734 (Emhart), however having a non semispherical front of the core pin. The technical problem of the invention is to make one of the cylindrical layers from recycled molten material ("post consumer scrap" or "flake") and still guarantee that the inner layer of first injected material is not penetrated and keeps a minimum thickness. For solving said problem an arrangement is suggested for manufacturing a preform according to the mentioned type and having at its entire inner surface an inner layer with a minimum thickness, which guarantees that the following recycled layer of the multi-layer does not reach near to the inner surface. The arrangement is outlined in claim 1. Possibly contained detrimental substances in the recycled material may therefore not reach the inner surface of the preform and therefore not the inner surface of a bottle, stretch-blown from said preform. The unround area is symmetrical to the central axis of the elongated core (claim 1) and placed in a half sphere surface of the core pin. Unround shapes of the front end of the core are a flattening or lower level flattening or a combination made from a ring shaped flattening and a centre dimple. The invention avoids with such suggestions that the recycled molten material, injected at the front end, opposite of said unround core tip, can reach the inner surface and secures that the recycled layer really stays "a center or outer layer" in the preform, not even touching or coming near the inner surface at the most vulnerable area of the preform during a production process. Providing such unround shape of the core pin, recycled material may contain amounts of contaminants that must not be poisonous but would prevent this recycled material from use at all, since even small amounts of contaminants would exclude such "recycled bottles" from use with food products. The invention provides that recycled material can be used for the preform and therefore for the stretch-blown container, by which use of recycled material raw material (virgin material) can be saved to make the bottle cheaper but still fully usable in food applications. The same object is solved by a procedure in which the front ended thinnest zone of the inner layer (in a preform) is moved from the middle axis into a ring zone at the core pin and an operating window of volume percentages is used for the sequentially injected materials (claim 10). The front ended area of the core forms the base section of the preform, respectively the container; the backwards area corresponds to the closing ("thread") section of the container, in which the cap is placed onto the container. To prove to a third party that such method with recycled material has been used and that a certain preform definitely incorporates flake (or recycled) material although not seen, the inventions suggests to use thermal monitoring (claim 11). Determining amount, distance and magnitude of peaks of temperatures being measured at the front end of the core, close to the unround front end, in particular in a blind bore opposite the flattening, can be used to provide real proof about the use of flake material for a certain preform (claim 13). The invention therewith makes use of temperature peaks, arising when introducing molten material for every layer of the preform. Depending on the injected material, the pressure and the layer (manufacturing step) the measured peaks alter. The rate of rise of the peaks may also be taken into account. Evaluating a plot of temperature T(t) over time (t) can exactly prove which material has been introduced for which layer of said preform and which time if compared to a model being stored in a computerized hardware (evaluation unit and comparing unit according to claims 12,13). Besides providing proof for the use of flake material such measurement can be used for quality assurance. The model may be contained as dot-model over time in the evaluation unit. Examples of the invention will be explained in more detail by the help of several embodiments. Figure 1 shows the semisphere front 2a of an elongated inner core 2 having a flattening 3b in the front end. Figure 2 shows an example of how a multi-layer preform may be manufactured by injecting molten plastics (following the arrow S) in a cylindrical Cavity, In this example five layers are used. Figure 3a, 3b, 3c show several designs of the front end 3b of mentioned core pin 2. Figure 3b having a front ended flattening 3b' and a shallow middle-dimple 3a. The radius dimensions are given by D 0 (core pin); D 1 (dimple) and D 2 (flattening). Figure 3c shows a hollow core pin with an identation having flat base 3b'' with a diameter of D5 and represents two alternatives of transition areas 3d,3e in left hand and right hand sketch (both sides of axis 100). Figures 3a to 3c may also be designed as hollow 2c core pins 2 to enable inner cooling. Figure 4 is an example of a preform 20 having substantially the same wall-thickness and having its most vulnerable area 14 at the bottom near the orifice 12. The thread part is 13. Figure 5 is an example of how the thermal measurement at the front ends 3b,3b',3b'' of the several core pin fronts 2a may be performed. Further examples of how a hollow cylindrical form may be performed and how the orifice may be designed may be taken from state of the art, e.g. EP-A 392 571 with its figure 1 or with respect to the injecting method its figures 2 and 3, or from EP-B 186 154 with its figure 4 or figure 5. Giving a general survey, reference is made to figure 2 . In said figure the production of a multi-layer preform is depicted schematically, by displaying a section through the corresponding injection mould 11 (outer body) and 2 (core) and by showing how the middle molten layer 10 generally advances in the cavity between core 2 and outer body 11. In the displayed example the advancing layer is the inner layer 10 being of material A, representing the virgin material. Before introducing said virgin material A, the scrap or recycled material B had been introduced through the orifice 12 and advanced through the center of the cavity. Before that, material A (virgin material) had been introduced to form the inner surface 8b and the outer surface 8a of the preform. The multi-layer preform of figure 2 is a five-layer preform being only one example of the invention. Also a three-layer preform may be manufactured as well as a two-layer preform. In figure 2 the front end 3b of the front 2a of the core pin 2 is designed as will be outlined in more detail in the other figures. Figure 1 shows a possible end flattening 3b of the front 2a of the core 2, which projects into the cavity 1 of the mould 11. Mostly, a couple of core pins are projecting into a couple of cavities, arranged close to each other in a larger block 11 being water cooled. The orifice 12 runs into the cavity 1 at its bottom end near to the front end 3b of the core 2. A central axis 100 is such defined, The orifice 12 usually is in the central axis and fed by two or more pipes, delivering different molten thermoplasts into the cavity. Figure 1 shows the second step of manufacturing, namely the introducing of recycled material 7 forming a middle layer in the virgin material A having been introduced before. By advancing the recycled material 7 two layers 6a,6b are defined, one at the inner surface of the outer mould 11 and one at the outer surface of the core 2. To avoid that recycled material 7 reaches the front 2a of the core being opposite to the injection orifice 12 the front end of the core is flattened 3b, as also shown in figure 3a, having a diameter of D2. The flattening 3b provides that the thinnest region of the inner layer 6a of virgin material A no longer is in the central axis 100 but in a circular area around mentioned central axis. There the inner layer 6a has the thickness of d 1 being thicker than the thickness of the inner layer 6a at the central axis 100 when using a full semi-sphere-shaped core pin. By such design of the front end of the core the displacement of the thinnest area with respect to place and form is effected (ring-form instead of central circular form and around the central axis instead of in the central axis 100 of the core). The unround design of the front end surface 3b of the core may provide that the thinnest region d 1 of the inner layer is thicker than the thinnest region when using known cores like the core 26 in figure 2 of EP-A 392 571. Figure 3b shows a dimple 3a within a flattening 3b'' at the front end of the core pin. Figure 3c displays a hollow (coolable) core pin 2 with semisphere front portion 2a, back portion 2b and inner coolant space 2c. The front end is designed in two examples, shown left and right hand of the axis, however to be understood as individual fully symmetrical. The flattening 3b'' has a diameter D 5 and a lower level than the front end and as the pure flattening 3b of figure 1. One intermediate smooth portion 3d may catch up to the outside surface by having two similar or equal radii D 3 and D 4 . Another possible intermediate portion 3c,3e introduces a circular bead D6, reaching even below the base level 3b''. Practically tested dimensions on a core pin of figures 3a and 3b are D 2 =7mm with a core pin diameter D 0 =20mm. In figure 3b the diameter D 2 is slightly larger than 7mm and D 1 is slightly smaller than 7mm. Practically tested dimensions are D 1 =4mm and D 2 =10mm with a core pin diameter of D 0 =20mm. In figures 2 and 4 the preform 20 after the manufacturing step 2 (still) shows three layers 6a,6b and 7, The middle layer 7 being made from recycled material B. It is neighboured to the inner and outer side by a layer of virgin material A (PET, PEN or polypropylen or nylon 66 or future material with comparable properties) so that material B does not touch the surface of the core pin 2. A danger that a (partly) contaminated inner surface of a preform may harm the food-purity of the containers, stretchblown therefrom, is thus remedied already when producing the preform. Virgin material can be saved by use of the recycled material without reducing the thickness of the inner securing layer of virgin material. An enlarging of the thickness at the vulnerable zone 14 of the preform 20 (opposite to the injection nozzle) is obtained. The mentioned "second step" of production is the step displayed in figure 1. In said step the scrap material B is pressed into the cavity and advances within the first material A along the preform to build up three layers 6a,6b and 7. To clean the nozzle from the scrap material and for preparing the step 1 of the next preform, step 3 (not shown in figur 1) is introduced and advances virgin material A through nozzle 12 again, The advance of said material A may, however, be short and only within the region of the front end of the core or may split up the recycled material 7 at the bottom of the preform into two recycled layers 9a,9b as shown in figure 2. The shape of the recycled layer 7 is therefore only depending on the volume of the virgin material introduced into the orifice-area 12 during step three. It will be evident that instead of using three layers 6a,7,6b in figur 1 only two layers 6a,7 may be used, the multi-layer preform then having an outer recycled layer. Equally, it is possible to use more than three layers, for instance, five layers as shown in figure 2. The main benefits of the third injection of virgin material A as being the third step of the manufacturing process of the preform is the purifying of the orifice 12. By such third step it is avoided that mixtures between material A and material B whithin the area of the orifice 12 are established. By such mixtures the recycled material would in lower concentration but still in a concentration large enough to harm the food-compatibility of the preform arrive at the inner surface. During tests the percentage of volume of a preform containing recycled material was placed into an operating window to give secure food-compatibility, Such operating window requires that material A as being the first layer should not be below about 45%, when material B is introduced with about 25% of volume. material A material B material A 45 % 25 % 30 % to to to 60 % 25 % 15 % If the operating window is neglected and virgin material A is introduced below 45% of volume, a danger arises that the scrap material introduced afterwards will reach either at the top 13 of the preform (near the thread) or at the bottom 14 of the preform 20 the surface of the core 2. If material B as scrap material is introduced to a larger extent than 25% it may also be too much of volume, introducing the danger mentioned before. If the amount of introduced material B, however, is substantially below 25% of volume, it will have only little bonding effect and the multiple layers may delaminate. The injection process of the third step of manufacturing should introduce a volume of virgin material not below 15%. By introducing said amount of virgin material three objects are achieved. Mainly, the nozzle 12 is cleared from scrap material B for providing a start condition for the next preform, which starts with introducing virgin material A. A further object of the third manufacturing step is the advancing of the first A-layer up to the thread-zone 13 of the preform to be on the safe side that the scrap material does not reach the surface on the threaded end and the virgin material A is safely within the whole thread-area. Finally, the third step of introducing virgin material is to dilute the bottom B-layer and advance the layer B further into the preform. The injection time depends on the pressure and the temperature. It also depends on the flattening or dimple shown in sketch in figure 3a, 3b, 3c. A test showed that satisfying results are obtained, when: material: injection time: temperature T: material A approx. 1,5 sec at 280 °C material B approx. 1 sec at 280 °C material A approx. 1,3 sec at 280 °C. With said injection time and injection temperatures no erosion or melting of the established (cooled) A-layer 6a resulted, when hot material B entered through the orifice 12. Figure 5 is a sketch of a water cooled core pin having a hollow space 2c and being cooled by turbulent water flow through a fountain cooling 2c',2c'' (arrows indicate water flow). The front end carries a section 31, which is made from copper. Around the central axis 100 the copper 31 has a flattening 3b. From the inside of the core pin 2 there is introduced a thermocouple 30 within a blind bore 31a. The thermocouple 30 is connected to a printer or plotter via a lead 30a. The length of the blind bore 31a is designed such that the thermocouple 30 will reach as close as possible to the bottom of the flattening 3b. In tests a suitable distance between the bottom of the flattening 3b and the end of the blind bore 31a has been evaluated to be 1.5mm. The inside surface of the inset or section 31 is thoroughly insulated with a thermally blocking cover 30b, to keep coolant effects off the measuring thermocouple end 30. In alternate embodiment the section 31 is reasonably isolated at its border surface to the remaining core. A temperature versus time plot T (t) shows peaks of the temperature, measured by the thermocouple 30 in figure 5. The peaks correspond to the input pressures and input temperatures of the hot (molten) thermoplast, introduced through the orifice 12 onto the front 2a of the core pin 2. By evaluating the peaks, which constantly decay from manufacturing step to manufacturing step, one is able to prove that a certain material has been used at a certain stage of the manufacturing process. For such evaluation a model is used within a not displayed unit to provide a reference model of how the temperature versus time should look like if a proper preform is produced. If the measured temperature at the thermocouple 30 deviates significantly from this model, one is able to tell that either quality has to be improved or that certain defects are within the amounts of molten material introduced into the cavity. Said plot or said evaluation of said peaks from mentioned plot are unconceivable proof to third parties that proper preforms had been manufactured and recycled material has been used within said preform but properly enclosed within layers of virgin material. The temperature measuring also enables to tell about a certain time, e.g. material A was introduced at a certain temperature to determine when the minimum volume (e.g. 45%) being necessary for a consolidated inner A-layer 6a is introduced.

Claims: Arrangement for manufacturing a preform from a thermoplastic melt, such as PET, PEN or polypropylen, for being further manufactured into a plastic container, especially a PET- or PEN-bottle, comprising an elongated cylindrical space (1) in which an elongated cylindrical core pin (2) having a front and a back portion (2a,2b) and an outer diameter (D0) being reduced compared to the inner diameter (d0) of the cylindrical space (1) is provided, such that between the inner surface of the cylindrical space (1) and the outer surface (D0) of the core pin (2) essentially the same distance (d0-D0) is provided over a large area including the area of the front portion (2a) of the core pin (2), the front portion (2a) of said core pin (2) being semispherical and having a front end surface being designed symmetrical with respect to the central axis (100) but un-round (3b,3b',3b'';3c,3d,3e); characterized in that the un-round front end surface (3b,3b',3b'';3c,3d,3e) comprises a ring zone flattening (3b') or an axially backwards oriented centre flattening (3b,3b''), the flattening (3b',3b,3b'') being oriented perpendicular to the central axis (100). Arrangement according to claim 1, the ring zone flattening (3b') having a centered and backwards oriented shallow indentation (3a), in particular a dimple. Arrangement according to claim 1, having a backwards oriented centre flattening as a base (3b'';D 5 ), being at a lower level than a remaining semisphere front of the core pin (2) and having (a) a smooth transition (D 3 ,D 4 ;3d) towards the remainder of the semisphere front (2a); or (b) an annular groove (3c;D 6 ) arranged between a smoth transition section (D3;3e) and the base (3b'';D 5 ) and having a lower level than the base (3b''). Arrangement according to claims 2 or 3, in which the radius (D 0 /2) of the core pin front portion (2a) is large compared to the radius (D 1 /2,D 2 /2,D 5 /2) of the shallow indentation (3a) or flattening (3b',3b,3b''), oriented backwards. Arrangement according to claims 2 or 4, in which the shallow indentation (3a) has a smaller radial extension (D 1 /2) than the ring zone flattening (3b'). Arrangement according to claim 5, in which the un-round front end extends in radial direction (D 1 ,D 2 ) up to no more than 1/4 to 1/2 of the radius (D 0 /2) of the front sphere portion of the core pin (2), but is only small in axial (100) direction. Arrangement according to any one of claims 1 to 6, in which (a) a thermal detector (30,30a) is coupled to the front end portion (2a) of the inner core pin (2;2a,2b) having a measuring section (30) introduced near to the un-round outer surface (3b,3b'3b'') of the front end portion (2a); (b) at least a central part (31) of the front end portion (2a) of the core-pin (2) is made from metal of high thermal conductivity, such as copper, silver or an alloy. Arrangement according to claim 7, characterized in that the thermal detector (30,30a) is constantly coupled to a long-term printing or plotting device for printing or plotting the temperature (T) over time (t) during moulding of the preforms (20). Arrangement according to claims 7 or 8, having an evaluation unit coupled to temperature data from said thermal detector (30,30a) to detect peaks (40a,40b) of the temperature data over time, the time intervals between said peaks (40a,40b) and the number of peaks for one moulding process of one preform, and having a comparison unit for comparing the temperature data with model-data contained in the evaluation unit. Method for manufacturing a plastic preform, in which through an injection orifice (12) at least three thermoplastic melts (8,9,10; A,B,A) are sequentially injected into a hollow cylindrical ring-space (1) having a ring-space volume between a core pin (2;2a,2b) and the inner wall of an outer form (11) and the zone of smallest thickness (d 1 ) of the plastic layer (6a) neighbouring the outer surface of the core pin (2;2a,2b) is displaced away from the central axis (100) of the core pin (2) into a ring-shaped zone around the central axis (100), characterized in that an operating window of volumes of the sequentially injected thermoplastic melts (A,B,A) is defined, to secure food compatibility of a container blown from said plastics preform; the operating window comprising injecting a first volume of virgin material (A) being not less than 45% of said ring-space volume prior to injecting a first volume of recycled material (B) substantially being 25% of said ring-space volume and then injecting a second volume of virgin material (A) being not less than 15% of said ring-space volume.

Inventors: PALMER PETER OAKFIELDS, TEVLIN JOHN, PALMER, PETER, OAKFIELDS, TEVLIN, JOHN
Applicants: SCHMALBACH LUBECA, SCHMALBACH-LUBECA AG
Application year: 1994
Main classification: B29C45
IPC codes: B29C45